Manganese is about twelfth in the list of the most abundant elements in the earth's crust, and there are more than 150 known minerals which contain manganese as an important constituent. An ore which contains at least 35% manganese is classified as a manganese ore. An ore which contains from 10-35% manganese is referred to as a ferroginous manganese ore. An ore which contains from 5-10% manganese is known as a manganiferrous ore.
An important class of manganiferrous ores are manganiferrous-silver ores which contain both manganese and silver. Manganiferrous-silver ores occure in the oxidized portions of the silver deposits of the western United States. In some such ores, silver is associated with manganese oxides which are dispersed through calcite causing the host rock to be colored black and known as black calcite. Black calcites can occur in carbonate rocks (having 10-1500 ounces of silver per ton) and in volcanic rocks (having less than 1 ounce of silver per ton).
Silver mineralization associated with manganese oxides and black calcites can occur in veins or as diseminations. In the United States, silver-bearing manganese veins are located in a broad arc that extends from southern Colorado to southwest New Mexico, southern Arizona, and southeast California. Areas of disseminated silver-manganese mineralization include the Creede and Silver Cliff districts of Colorado and the Tombstone and Hardshell districts of Arizona. These deposits are also characterized in having significant amounts of silver halides: cerargyrite (AgCl), bromargyrite (AgBr), and iodargyrite (AgI). Black calcites are also very common in these deposits.
Oxidized silver ores containing the higher oxides of manganese are generally refractory to hydrometallurgical methods of treatment. Ores with a high proportion of siver and lead have been smelted in the past to obtain these metals. Ores with a low percentage of silver but with a high percentage of manganese and iron oxides have been used for making spiegeleisen and ferromanganese. When ores have contained lower silver, lead, manganese, and iron content, they have been smelted whereby manganese and iron passed into the slag and silver and lead were recovered. In the past, when these ores could not be smelted, they were generally treated by cyanidation with resultant low efficiency in the recovery of the metals.
The United States Bureau of Mines studied the problem of treating silver-manganese ores in the early 1920's, and it was found that the refractory silver in the original ore was insoluble in all the common solvents for metallic silver and its salts. (Clevenger, G. H. and Caron, M. H., "Treatment of Manganese-Silver Ores", U.S. Bureau of Mines Bulletin, B226, 1925). Solvents in which the silver in the original ore was insoluble include: cyanide solutions of all concentrations, dilute and concentrated nitric acid, dilute sulfuric acid, salt solution, alkaline thiosulfates, ammonia, mercury, and other reagents. The conclusion was that in order to obtain the highest recovery of silver, all of the manganese had to be dissolved, or the higher oxides of manganese had to be reduced to manganous oxide (known as the Caron Process when reduction is done pyrometallurgically).
Wet chemical methods to reduce higher manganese oxides were also studied in the Bureau of Mines study. These wet methods included: wet sulfidizing with preliminary treatment of silver manganese ore with hydrogen sulfide; treatment with sulfurous acid using an aqueous solution of sulfur dioxide; treatment with metallic iron; and treatment with pyrite. The reduced ores were subsequently treated with cyanide to recover silver.
Clevenger and Caron (1925) did not have enough information on the mineralogy of silver-manganese ores, and their attempt to determine the form of silver in the silver-manganese ores that were examined was unsuccessful.
Other methods have been disclosed in the prior art for extracting silver from silver-manganese ores. Such methods include: a chloridizing roast; extraction with thiosulfate solution; treatment with sulfur dioxide; a two step extraction process wherein the second step involves an extraction with cyanide; direct cyanidation; malonitrile leach; and leaching with ammonium hydroxide-sodium thiosulfate under slight oxygen pressure.
The various prior art methods of extracting silver and manganese from silver-manganese ores have one or more of the following problems: they are inefficient without recovering a satisfactory amount of silver and/or manganese; they are too expensive to operate, e.g. they require neutralization in shifting from an acidic to basic environment and/or they require two or more ore extraction steps; they require the use of toxic materials, e.g. cyanide.
In the United States, there are vast reserves of silver-manganese ores, but there is no efficient method disclosed in the prior art for extraction of the valuable metals from those ores.